Narrowband Emitters with Fast Reverse Intersystem Crossing and Suppressed Concentration‐Quenching Enabled by Engineering the Donor in a Through‐Space Charge‐Transfer System with A Multiple Resonance Acceptor

ABSTRACT Multiple resonance (MR) emitters demonstrate great potential for fabricating narrowband organic light‐emitting diodes (OLEDs), which, however, show low reverse intersystem crossing (RISC) rates ( k RISC ) and severe concentration‐quenching. Here, indolo[3,2,1‐ kl ]phenoxazine (IPXZ), indolo[3,2,1‐ kl ]phenothiazine (IPTZ), and indolo[3,2,1‐ kl ]phenoselenazine (IPSeZ) are developed as donors for a typical MR acceptor (DtBuCzB) in a through‐space charge transfer system, affording MR‐TADF materials ( BN1 , BN2, and BN3 , respectively) with high k RISC and largely suppressed concentration‐quenching. Close, cofacial donor/DtBuCzB alignments are formed within the emitters, which effectively accelerates the RISC and protects DtBuCzB from aggregation, especially for BN2 / BN3 with S / Se ‐containing donors. In the 2 wt.% doped films, the emitters exhibit narrowband blue‐green emission with high k RISC values up to 5.0 × 10 5 s −1 , which are markedly enhanced compared to that of DtBuCzB (1.0 × 10 4 s −1 ). In the 5–20 wt.% doped films, the emitters maintain narrowband emission and show high photoluminescence efficiencies, and the intermolecular donor/DtBuCzB interactions further enhance the k RISC values (up to 10 6 s −1 ). Non‐sensitized narrowband OLEDs using the emitters show maximum external quantum efficiencies (EQEs) up to 35.2% and EQEs at 1000 cd m −2 up to 26.1%. The maximum EQEs remain up to 27.1% at the 20 wt.% doping concentration.